The present invention relates to a plain bearing, more particularly to the plain bearing for internal combustion engines.
There has been known a plain bearing comprising a back metal layer, a bearing alloy layer made of a copper alloy or an aluminum alloy, and an overlay which is provided on the bearing alloy layer in order to enhance conformability and foreign-matter embeddability of the bearing.
There has been also well known an overlay Pb-base alloy comprising Sn and In. For example, JP-A-4-2739 teaches a tin-base alloy comprising 0.1 to 25 wt % In. According to the publication, the tin-base alloy has excellent corrosion resistance derived from Sn and can solve the problems of tin which lacks conformability and affinity for oil and which has a high coefficient of friction.
Recently, however, a lead-free plain bearing is sought as a countermeasure against the environmental problems, so that an alternative overlay made of a lead-free alloy is sought. On the other hand, regarding internal combustion engines, there is a recent trend that they are provided with a high performance of high speed and high load, so that a plain bearing especially having excellent properties of anti-seizure and fatigue resistance is highly sought.
It has been generally known that the tin-base alloy overlay has excellent corrosion resistance in comparison with the lead-base alloy overlay but the former is inferior in anti-seizure and fatigue resistance properties than the latter. It is also noted that the Sn-In alloy disclosed in JP-A-4-2739 has a considerably low melting point because of the additive alloying element of In (indium), so that the Sn-In alloy will not be so suitable for plain bearings of internal combustion engines, which are used under a high temperature of around 130xc2x0 C. to 150xc2x0 C., because of rather unsatisfactory anti-seizure and fatigue resistance properties.
Thus, it is an object of the invention to provide an overlay for plain bearings, which has excellent high temperature properties of anti-seizure and fatigue resistance.
According to the present invention, there is provided a plain bearing comprising a back metal layer, a bearing alloy layer and an overlay which coats the bearing alloy layer, wherein the overlay is made of a lead-free tin-base alloy which comprises, by weight, from more than 2% to 10% Ag. The plain bearing has excellent anti-seizure and fatigue resistance properties. The bearing alloy layer may be of known bearing alloys including copper alloys and aluminum alloys. The plain bearing can be lead-free in the case where the bearing alloy layer is made of a lead-free alloy.
Regarding the chemical composition of the overlay alloy, if the Ag amount is not more than 2 wt %, the above advantageous effects of excellent anti-seizure and fatigue resistance properties can not be obtained. The overlay alloy is the most excellent at an amount of about 3 wt % Ag with respect to anti-seizure and fatigue resistance properties. If the Ag amount is increased over the level of about 3 wt %, the both properties are gradually deteriorated corresponding to the increased level and extremely deteriorated at an Ag amount of more than 10 wt %. Therefore the Ag amount is desirably 2.5 wt % to 5 wt %.
The overlay may be formed on the bearing alloy layer directly or through an inter layer made of Ni or Ag in order to avoid occurrence of the mutual diffusion or bonding problem between the bearing alloy layer and the overlay.
The overlay may optionally comprise other alloying elements than Ag in order to further improve mechanical properties thereof, especially fatigue resistance. Such alloying elements may be preferably Cu, Sb, Zn, Ni and so on because the melting point of the overlay is not lowered thereby.
The overlay may also optionally comprise hard particles such as nitrides, carbides or oxides in order to improve wear resistance property, or solid lubricants such as MoS2, BN, graphite, PTFE and so on in order to decrease the coefficient of friction.
The invention plain bearing is excellent in high temperature properties of anti-seizure and fatigue resistance, and especially suitable for internal combustion engines.
Herein below a detailed description will be provided with regard to invention and comparative examples.